Broadband wireless systems such as Local Multipoint Distribution Systems (LMDS), known as Local Multipoint Communication System (LMCS) in Canada, are being developed to provide point to multipoint, high bandwidth services between a base station connected to a backbone such as an asynchronous transfer mode (ATM) network and network interface units (NIUs) at fixed or mobile locations within a defined geographic area known as a cell. A wireless link between the base station and the NIUs operates at a wireless radio frequency (RF) typically in the 28 GHz range depending on the allocated frequency license. A transceiver at the base station and a transceiver at each NIU site supports bi-directional, broadband “last mile” communication between a service provider and a customer.
Traditional wireless access systems employ one polarization or another (vertical or horizontal, for example) as a means for delivering services over a radio medium to a given customer(s) site. These systems tend to be optimized for specific types of services that are largely dictated by the radio licensing structure and/or regulatory requirements. With the advent of broadband licensing (LMDS/LMCS, for example), large numbers of different service types can be offered using a common delivery infrastructure. These varying services can be low bandwidth in nature (so called POTS, T1 or E1, fractional T1 or E1, Ethernet, or others, for example) or can be high bandwidth in nature (so called T3 or E3, OC_n, or others, for example). Typically, the low bandwidth services are more cost effectively delivered through the sharing of radio resources. This can be achieved by sharing the radio resources in time, for example, using techniques such as time division multiple access, (TDMA). This technique divides a given radio-communication channel up into time slots which are allocated in a fixed or dynamic manner to the various customer site equipment which are sharing this radio channel/resource. Although this tends to be more cost effective, this type of access technique commonly employs lower efficiency modulation schemes, quadrature phase shift keying (QPSK), for example, which utilize more spectrum/license.
Typically the high bandwidth services are not as cost sensitive but demand much a more capacity and therefore need to be connected using high efficiency modulation techniques, quadrature amplitude modulation (QAM), for example. These are not amenable to radio resource sharing and therefore are more optimally run within independent radio channels. The technique of using a number of independent radio channels serving one customer site each is referred to as frequency division multiplexing (FDM).
The frequencies available for RF wireless communication are limited and there is an economic incentive to make the best use of the band of frequencies covered by a particular frequency license. Prior art Radio Frequency (RF) wireless networks have been designed to optimally utilize the allocated RF Spectrum, maximize capacity and to minimize the cost of the system. Different solutions have been proposed for various applications.
In a cellular system a typical cell is configured to provide service to a geographic area. The cell, often described generally as a circular area, has a more or less central base station or hub with the necessary hardware to conduct point to multipoint, downstream communication with user stations within the cell. Each user station is also provided with the hardware including a directional transceiver for conducting point to point upstream communications with the base station. Depending on the application, the cell may be divided into sectors and sectored antennas are situated at the base station site to provide restricted communication within a particular sector. A typical cell may have a diameter in the 3 to 5 km range.
For a large geographic area, such as a metropolitan area, a number of similar cells are laid out in a slightly overlapping configuration to provide complete coverage. Each cell has its own base station or hub, possibly with a sectored antenna, to subdivide the cell into a plurality of sectors. In an ideal situation each cell employs the same frequency band for each sector in a base station to user site direction, known herein as downstream (D/S), and another frequency band within each sector for transmission from the user site to the base station (upstream or U/S). When more than one cell is required to cover an area, however, inter-cell interference caused by the common frequency band becomes a significant problem. One method of overcoming this form of interference is to use different frequency bands in each cell. Because of frequency licensing restrictions a service provider has a limited frequency range with which to work and consequently is unlikely to have sufficient frequency bands to provide different frequencies for each cell. Consequently, it is important to make efficient re-use of frequency sets (U/S and D/S) in cells within a geographic area.
For fixed wireless point-to-multipoint systems, a common network configuration is the straight grid design, wherein a number of central hubs (base stations) are located at the intersection points between perpendicular imaginary gridlines. These hubs communicate through radio-wave propagation over the air to a multitude of peripheral transmit/receive units or Customer Terminals (CTs) that may be fixed or mobile in nature.
Since a multitude of RF signals may be transmitted simultaneously in the network coverage area there is a risk that interference will occur where the desired signal or Carrier (C) is drenched by the Interfering signal(s) (I). Here C and I would typically (but not always) be of the same frequency. To mitigate interference, i.e. to isolate the desired signal from interfering signals, various measures have been developed, for instance, by using time division, antenna polarization, coding or spatial separation.
Spatial separation is widely used but has the disadvantage that the utilization of the RF Spectrum suffers. A metric called Frequency Re-use is used to quantify how effectively the allocated spectrum is used.
Polarization diversity may also be employed in sectored cells to improve frequency re-use. In co-pending U.S. application Ser. No. 09/073,217 to Boch, the contents of which are incorporated herein by reference, adjacent sectors employ orthogonal polarization to reduce cross-polar interference. Additionally, upstream transmission (customer site to base station) and downstream transmission (base station to customer site) employ orthogonal polarization to reduce co-polar interference.
U.S. Pat. No. 5,949,793 which issued Sep. 7, 1999 discloses a sectored cell configuration for transmission of broadband programming such as TV plus digital communications services. The '793 patent also employs polarization diversity between adjacent sectors. The '793 patent also illustrates without detailed description the concept of skewing sectors with respect to sectors in adjacent cells.
Polarization diversity is also discussed in U.S. Pat. No. 5,809,431 to Bustamante et al and in U.S. Pat. No. 5,838,670 to Billstrom.